Edge inside church (4)
I believe the answer is:
inch
'edge' is the definition.
(I've seen this before)
'inside church' is the wordplay.
'inside' becomes 'in'.
'church' becomes 'ch' (common abbreviation).
'in'+'ch'='INCH'
(Other definitions for inch that I've seen before include "Measure of length" , "Rare bird follower" , "Creep, edge" , "Progress slowly" , "Edge slowly" .)